When comparing FuelPHP vs Slim, the Slant community recommends Slim for most people. In the question“What are the best PHP frameworks?”Slim is ranked 1st while FuelPHP is ranked 10th. The most important reason people chose Slim is:

Slim's documentation is well organized and detailed, every concept is thoroughly explained and it is very helpful for both advanced users and beginners.

Pros

Pro

Total flexibility

Fuel has very few restrictions on how to write code. Classes and controllers can be in any file structure, any folder can be the "modules" folder and native classes can be extended any way you want.

Pro

License

FuelPHP is open source and is released under the MIT license.

Pro

Out of the box HMVC structure

FuelPHP follows the HMVC pattern which makes it possible to divide the code into smaller modules.

Pro

Secure

Fuel takes security very serious, and as a result, has implemented the following measures to ensure the safety of your web applications:

Output encoding in views

CSRF protection

XSS filtering

Input filtering

SQL injection

Pro

Powerful yet lightweight ORM

FuelPHP is all about being lightweight and simple, this is also demonstrated by it's built-in ORM, it's simple yet powerful. It maps a model to each table in the database, assigns fields on the table depending on the model configuration.

Pro

Well organized and thorough documentation

Slim's documentation is well organized and detailed, every concept is thoroughly explained and it is very helpful for both advanced users and beginners.

Pro

Open source

The Slim Framework is open source and is released under the MIT public license

Pro

Hooks for executing code at different points in its life-cycle

Slim supports code hooks for executing functions at different points in time during the application's lifecycle.

Pro

Supports tie-ins for Rack-like middleware

Rack is an interface used in Ruby frameworks used to group and order modules, which most of the time are Ruby classes, and specify between them.

Slim uses a simple concept for it's middleware. By wrapping HTTP requests and responses it unifies the middleware into a single method call.

Pro

Useful classes

Pro

REST based

REST fans will love the REST based architecture.

Pro

A good starting point

Slim is minimal and that is a good thing if you want to start from there. It can be easily extended and even supports popular packages that are used in Laravel (like Illuminate\Database (eloquent)) for example.

Pro

Supports Php 5.3 and PHP 7

Pro

Flexible

Slim doesn't demand that you stick to a fixed folder structure. As long as you load Slim the right way you can do anything from there the way you like it.

Cons

Con

Small community

FuelPHP has a pretty small community compared to other alternatives. This means that there are not many resources and guides for it out there, but on the other hand, it's easier to get help from the core team of developers that are working on Fuel.

Con

Very little consistency among different versions

There have been quite some changes that break the compatibility between Slim 2 and Slim 3. Even if you learned how to work with the Slim 2, you will find that Slim 3 requires re-training.

Con

Too minimal

While it's true that Slim is a microframework, it's still too minimal. When used for throwaway projects or simple prototypes, it's perfect. But in the long run, it becomes less and less useful and you end up in implementing a full custom framework in trying to tackle all the missing features.